1. Field of the Invention
The present invention relates to a hydrodynamic bearing in which a shaft member is constructed by joining and fixing a disc member such as a disc-shaped thrust plate to a shaft and a method of manufacturing the same, an apparatus of manufacturing a shaft member for a hydrodynamic bearing, a spindle motor having the hydrodynamic bearing, and a recording disk driving apparatus for rotating a recording disk by a spindle motor.
2. Background Information
Hitherto, as a bearing of a motor for rotating a recording disk in a hard disk driving apparatus, a removable disk driving apparatus, or the like, various hydrodynamic bearings for rotatably supporting a rotor by generating a dynamic pressure by a lubrication fluid such as oil held in a space between a shaft and a sleeve at the time of rotation of the motor have been proposed. Such a hydrodynamic bearing is constructed by, for example, a radial hydrodynamic bearing part and a thrust hydrodynamic bearing part. A shaft part (shaft member) is constructed by a shaft having a cylindrical outer circumferential surface and a thrust plate (disc member) provided so as to be substantially orthogonal to the axis of the shaft. The radial hydrodynamic bearing part is constructed in the outer circumferential surface of the shaft, and the thrust hydrodynamic bearing part is constructed in the flat surface of the thrust plate.
As a method of fixing the shaft and the thrust plate to each other, for example, a method disclosed in Japanese Unexamined Patent Publication (JP-A) No. 2000-324753 is known. In the method, the thrust plate is formed in an annular shape and is press fit to the outer circumferential surface at one end of the shaft, and a joint part between the end surface of the shaft and the thrust plate is fixed by laser welding. There is another known method disclosed in JP-A No. 2003-097545 in which a screw part is provided for each of the outer circumferential surface of an end portion of a shaft and the inner circumferential surface of an annular thrust plate and the screw parts are screwed to each other, thereby fixing the shaft and the thrust plate to each other.
In recent years, application of a recording disk driving apparatus used for an apparatus such as a personal computer to a smaller portable information terminal has been started. A spindle motor mounted on a driving apparatus of this kind is requested to have high-speed and high-precision rotation which are conventionally demanded and, in addition, smaller size, reduced thickness, lower cost, and lower power consumption.
To address such requests, however, the dimension in the axial direction of the shaft has to be shortened. In the case of using both press fitting and laser welding disclosed in JP-A No. 2000-324753 and the case of fixing the shaft and the thrust plate by screwing disclosed in JP-A No. 2003-097545, to maintain the perpendicularity of the thrust plate to the axis of the shaft with high precision, the thrust plate has to be thick to a certain extent. Consequently, it is difficult to sufficiently assure the support length of the shaft by the radial hydrodynamic bearing part.
In a spindle motor, holding of the posture of the rotor such as whirling during rotation of the rotor on which a recording disk is mounted thoroughly depends on the radial hydrodynamic bearing part. Therefore, to stably hold the posture of the rotor, the support length of the shaft by the radial hydrodynamic bearing part has to be sufficiently assured. It is, however, very difficult to make the motor smaller and thinner as a whole while maintaining the requested rotation precision.
JP-A No. 2002-168240 discloses a method of integrally forming a shaft and a thrust plate in order to reduce the thickness of the thrust plate. JP-A No. 2003-056567 discloses a method of fixing a shaft and a disc-shaped thrust plate by performing resistance welding on the axial part.
According to the method of integrally forming the shaft and the thrust plate (JP-A No. 2002-168240) and the method of fixing the shaft and the disc-shaped thrust plate by resistance welding (JP-A No. 2003-056567), the motor can be made smaller and thinner as a whole while maintaining the requested rotation precision. However, there are still the following technical problems.
Consequently, a molding method by casting is not suitable for a member as a component of the hydrodynamic bearing, such as the shaft and the thrust plate. The reason is that a number of small holes are formed in the surface of the member by casting and, in the case of performing a finishing process by cutting the surface of the shaft or thrust plate (surface precision process) after the casting process, metal particles such as powders generated at the time of cutting enter the small holes and cannot be easily removed perfectly by cleaning. If the hydrodynamic bearing is used while the metal particles such as powders remain on the surface of the members, the metal particles in the small holes are gradually raked out by the flow of a lubrication fluid by rotation and are mixed into the lubrication fluid, so that it causes problems such as seize, damage, and the like of the bearing.
Consequently, a method of forming the shaft and the thrust plate by a cutting process of, for example, cutting a metal rod member is common. In the case of forming the shaft and the thrust plate integrally as disclosed in JP-A No. 2002-168240, a metal rod member having a diameter larger than that of the thrust plate extended in a flange shape from the outer circumferential surface of the shaft is cut, so that long time is required for the processing, the yield deteriorates, and productivity decreases. Since the member is much wasted, it may disturb reduction in the cost.
In addition, in the case of integrally forming the shaft and the thrust plate, when a finishing process is performed on the flat surface of the thrust plate as a component of the thrust hydrodynamic bearing part, a grinding relief for finishing the bearing surface by grinding has to be provided for the root portion of the shaft (the corner portion between the shaft and the thrust plate). The grinding relief is created by a process of preliminarily removing a portion with which a grinding stone does not come into contact, of the corner in the root portion. Consequently, to assure the area of the flat surface of the thrust plate and obtain necessary support rigidity in the thrust direction, the diameter of the thrust plate has to be also increased. Therefore, the viscous resistance of the lubricant fluid at the time of rotation in the radial hydrodynamic bearing part and the thrust hydrodynamic bearing part increases, and the rotation load of the motor increases, so that a power consumption amount also increases.
In the method of fixing the shaft and the disc-shaped thrust plate by resistance-welding the axial portions of them (JP-A No. 2003-056567), problems such as increase in the cost and power consumption amount like in the case of integrally forming the shaft and the thrust plate can be avoided. However, since the resistance welding is performed in a state where a projection provided around the axis of the shaft and the thrust plate are in point-contact with each other, the stress and heat occurring when the shaft and the thrust plate are pressurized and pressed against with each other are concentrated in the projection portion of the shaft. It is therefore feared that a deformation such as deflection occurs in the case of using a thin thrust plate. Further, since the resistance welding is performed around the axis of the shaft, it is difficult to assure a sufficient welding area and it is also feared that the welding strength varies. If a heavy voltage is applied or current carrying time is increased to prevent such variations in the welding strength, new problems occur such as melting of an electrode contact portion between the shaft and the thrust plate and occurrence of so-called weld dusts created when the temperature in the welded portion becomes too high and a melted metal drifts.
When deflection occurs in the thrust plate as a component of the thrust hydrodynamic bearing part, naturally, the support precision deteriorates, so that the thrust plate cannot rotate stably. Since the weld dusts cannot be completely removed even by cleaning like the metal particles, it may cause seize or damage in the bearing part.
Further, in the case of joining the shaft and the thrust plate to each other by resistance welding, to eliminate the influence of variations in dimensional precision due to process tolerances and to join the members at high precision to an extent that it does not exert an influence on the axis support, it is necessary to absorb the variations in dimensional precision by an axis adjusting jig for adjusting the axes of the shaft and the thrust plate at the time of welding.
The problems as described above occur not only in the case of joining the thrust plate as a component of the thrust hydrodynamic bearing part to the shaft as a component of the radial hydrodynamic bearing part but also in the case of joining another disc member such as a disc-shaped member for preventing coming off to the shaft as a component of the radial hydrodynamic bearing part.